Interior permanent magnet (IPM) machines have a number of operating characteristics that make them attractive for use in vehicle propulsion applications. Compared, for example, to AC induction and DC motors, IPM motors can provide high efficiency, high torque and high power densities. IPM machines also have a long constant power operating range. An IPM machine typically includes a stator with multiphase windings. A rotor having interior permanent magnets is separated from the stator by an air gap. A magnetic field, produced by the flow of current through the stator windings, interacts with a magnetic field produced by the rotor magnets, thereby causing the rotor to rotate.
Permanent magnets have low permeability and therefore exhibit high reluctance directly along a magnetic axis (d-axis) inside an IPM machine rotor. Along a q-axis, between the magnetic poles or magnet barriers of an IPM rotor, there exists no magnetic barrier, and so magnetic reluctance is very low. This variation of reluctance around a rotor provides saliency in the rotor structure of an IPM machine. This saliency causes the rotor to tend to align with a rotating magnetic field induced by the stator. Thus an IPM rotor exhibits reluctance torque in addition to permanent magnet torque generated by magnets inside the rotor. Reluctance in a d-axis can be produced by one magnet per pole, for example, as utilized in single-barrier rotor designs. Reluctance in d-axis can also be produced with multiple barriers, where magnets are placed in one or more barriers.
Due to slotting effects between rotor and stator, the rotor of an interior permanent magnet (IPM) machine is subject to flux variation in the vicinity of the air gap as the rotor spins. Flux variation causes eddy currents to be induced in the rotor and the magnets, especially near the surface of the rotor, and can result in rotor losses and magnet heating. For high-frequency operation, for example, in many automotive variable speed drive applications, eddy current losses can make the magnet vulnerable to demagnetization. To prevent demagnetization, a common industry practice is to break the magnet into smaller segments along the axial length of the machine, thus increasing the resistance to eddy currents. This process, however, can make the rotor manufacturing more complicated where a large number of magnet segments are required to be inserted into the rotor.